With the advent of less expensive semiconductor memory, modern computer and microcomputer systems have been able to use bit-mapped video displays for the output of data from the system. As is well known, a bit-mapped display requires a memory which can store at least one binary digit (bit) of information for each picture element (pixel) of the display device. Additional bits stored for each pixel provide the capability of the system to render complex images on the video display, such as multi-color images, and background and foreground images, such as a graphics background with textual information overlaid thereupon. The use of bit-mapped storage also allows for data processing operations to easily generate and modify the stored image.
Modern video display devices are often of the raster-scan type, where an electron gun traces horizontal lines across the display screen in order to generate the displayed pattern. In order for a displayed raster scan image to continue to be displayed on the video screen, the image must be refreshed at periodic intervals. A common refresh rate for the cathode ray tube video display devices is 1/60 of a second, since the refresh operation carried out at that speed is not noticeable to the human user of the system. However, as the number of pixels displayed on a screen increases, in order to increase the resolution of the displayed image, more and more bits of information must be accessed from the bit-mapped memory in the refresh interval. If the bit-mapped memory has but a single input and output port, the percentage of time during which the data processing unit can access the bit-mapped memory decreases with the pixel size of the display if the refresh interval remains constant. In addition, the speed of the memory must increase, since more bits must be output during a fixed period of time.
Multiport random access memories have been developed which provide for high-speed output of data to the video display and also for increased accessibility of the memory contents to the data processing device. The multiport memories accomplish this by having a first port for random access and update of the memory by the data processing unit of the computer system and a second port for serial output of the memory contents to the video display independent from and asynchronous with the first port, thereby allowing access to the memory contents during output of data to the video display terminal. Examples of multiport random access memories are described in U.S. Pat. No. 4,562,435 (issued Dec. 31, 1985), U.S. Pat. No. 4,639,890 (issued Jan. 27, 1987), and U.S. Pat. No. 4,636,986 (issued Jan. 13, 1987), all assigned to Texas Instruments Incorporated.
In each of these prior multiport memories, data is shifted from some or all of the memory cells in a row of the random access array into a register, during a special transfer cycle. Serial output is then accomplished from the register in a manner which is independent from and asynchronous with the operation of the random access of data in the array. Serial input capability can also be provided in such devices, with another type of transfer cycle capable of shifting the contents of the serial register into a selected row of the random access array.
The serial "side" of these prior multiport memories has been constructed according to various architectures. For example, the device described in said U.S. Pat. No. 4,639,890 has a shift register as the register on the serial side, with the serial output beginning from a selected cell in the shift register from taps included therewith. Each serial clock pulse shifts the data along the shift register, with the output coming from the tapped shift register cell, to provide a serial stream of data. Serial input can of course be accomplished by providing input data to the tap point and shifting the input data stream along the shift register. If fewer tap points than cells are provided for the shift register in this device, however, flexibility of the starting point for serial output (and input) is compromised.
Greater flexibility for the starting point of serial input/output is provided by the device described in said U.S. Pat. No. 4,636,986, where a non-shifting register contains the data to be serially output. In this arrangement, a counter stores an address from which serial output is to occur, and a decoder acts responsive to the counter to select the one of the register cells from which serial output, for example, is to occur. Each pulse of the serial clock signal causes the counter to increment its stored value, with the decoder enabling the next register cell in sequence accordingly, in order to provide the serial data stream. Serial input is similarly accomplished, with the serial clock incrementing the register cell location receiving the serial input bit.
While the use of the counter/decoder architecture provides for increased flexibility with respect to the starting points for serial input and output, the counter and decoder circuits required to select, and to update the selection of, the serial register bit include built-in delays. For example, in order to increment the serial register position, the counter must increment its contents responsive to the serial clock pulse, and the decoder must again decoder the output of the counter before the next serial register cell is selected. Such delays, while they may be minimized by design and manufacturing techniques, are inherent in this particular architecture.
It is therefore an object of this invention to provide a pipeline architecture for the serial side of a dual-port memory, in order to improve the speed of serial output therefrom.
It is a further object of this invention to provide such a pipeline where the pipeline is disabled for purposes of serial input, so that serial input data is stored in the proper location in the serial register.
It is a further object of this invention to disable the pipeline for output during selection of another serial register location.
Other objects and advantages of this invention will become apparent to those of ordinary skill in the art having reference to the following description, along with the accompanying drawings.